1. Field of the Invention
This invention relates to an intake system for a multiple-cylinder engine, and more particularly to an intake system for a multiple-cylinder engine comprising a plurality of discrete intake passages which are connected to respective cylinders at their downstream ends, an integrated chamber into which the upstream ends of the discrete intake passages are merged, and an upstream side intake passage which communicates the integrated chamber with the atmosphere.
2. Description of the Prior Art
Recently, in an engine for a vehicle, it has been proposed to increase the charging efficiency by the use of kinetic effect of intake air in the intake system. In such a case, an integrated portion such as a surge tank to which an upstream side intake passage from an air cleaner is connected is provided in the intake system, and discrete intake passages branch off from the integrated portion and communicate with respective cylinders. In such an intake system, negative pressure wave generated in the downstream end portion of the discrete intake passage upon opening of the intake valve is inverted into positive pressure wave at the integrated portion which functions as a space open to the atmosphere and the engine is supercharged by virtue of the positive pressure wave, whereby the charging efficiency is improved and the engine output power is increased.
In the conventional system, as the integrated portion, there has been wide used a surge tank type integrated portion in which the upstream side intake passage is connected to one end face of the integrated portion or the center of one side face of the same, and the discrete intake passages are connected to the side faces of the same. In such an integrated portion, the distances between the junction of the upstream side intake passage to the integrated portion and the junctions of the respective discrete intake passages to the integrated portion inherently differ from each other and/or the lengths of the respective discrete intake passages inherently differ from each other. Accordingly, distribution of intake air to the respective cylinders cannot be uniform, and the kinetic effect of intake air cannot uniformly act on the respective cylinders. Further, since the flow path of intake air from the junction of the upstream side intake passage to the junction of each discrete intake passage makes a sharp bend, intake resistance of intake air increases.
In order to overcome these problems, there have been proposed various intake systems.
For example, the intake system disclosed in Japanese Unexamined Utility Model Publication No. 60(1985)-88062 has an integrated portion A as shown in FIG. 18. The integrated portion A is substantially a truncated cone in shape. The upstream side intake passage is connected to the end face of the integrated portion having a smaller diameter, and the discrete intake passages C are connected to the end face of the same having a larger diameter. B denotes the opening of the upstream side intake passage into the integrated portion A. The openings of the discrete intake passages C are arranged in the larger diameter end face symmetrically about axis L--L passing through the center of the opening B. With this arrangement, the distances between the opening B and the openings of the respective discrete intake passages C can be substantially equal to each other, and distribution of intake air to the respective cylinders can be uniform. Further, the flow path of intake air can be substantially straight and intake resistance is reduced. Further, since the openings of the respective discrete intake passages C into the downstream side end face are disposed near to each other, each discrete intake passage C functions as a space open to the atmosphere for the other discrete intake passages with respect to inertia effect of intake air and accordingly, the volume of the integrated portion A may be small.
In the intake system disclosed in Japanese Unexamined Utility Model Publication No. 57(1982)-101367, as shown in FIGS. 19 and 20, the opening E' of the upstream side intake passage E into the integrated portion D uniformly overlap with the openings F' of the discrete intake passages F into the integrated portion D. With this arrangement, it is expected that intake air introduced into the integrated portion D from the upstream side intake passage E is more uniformly distributed to the discrete intake passages F.
However, in the case of the integrated portion A shown in FIG. 18, the intake resistance cannot be sufficiently reduced depending on the shape of the integrated portion A, the relation between the areas of the end faces, the distance between the end faces and the like. Further even if the openings of the discrete intake passages F and the opening of the upstream side intake passage E are positioned relative to each other in the manner shown in FIG. 20, it is difficult to smoothly distribute intake air so long as intake air is distributed to many discrete intake passages from a single upstream side intake passage at one time.